In drilling wells by rotary methods it is a common practice to circulate, continuously, a drilling mud or fluid into and out of the borehole during the drilling operation. The drilling mud is pumped into the drill pipe from a mud pit and the mud passes down to the bottom of the borehole. The drilling mud then flows upwardly through the annular space between the borehole wall and the drill pipe, and finally flows from the borehole through a mud ditch back to the mud pit, wherein the mud is mechanically or chemically treated before recirculation through the borehole.
The drilling mud serves several purposes that influence such factors as the drilling rate, cost, efficiency and safety of the operation. The drilling mud lubricates and cools the drill bit, acts as a vehicle to carry the cuttings from the borehole, and provides sufficient equalizing hydrostatic pressure against the formation wall to prevent the borehole wall from cave-in during drilling. By using proper mud formulations, the borehole entry of gases and fluids encountered in the formations pierced by the drill is inhibited and possible collapse or blowouts resulting from uncontrolled influxes of these formation fluids may be prevented. The drilling mud also exerts a "wall-building" effect whereby it often forms a thin filter cake on the borehole wall, thus sealing off the borehole and reducing water loss to the penetrated formations.
An acceptable mud must have body yet be free-flowing with a relatively low viscosity in order to facilitate pumping. The mud must also have an acceptable gel strength in order to suspend solid material if circulation is interrupted and to prevent accumulation of solids at the drill bit to avoid mechanical jamming. Acceptable drilling muds may be either oil-based or water-based, and they are normally treated to provide the rheological properties that make them particularly desirable and useful for drilling wells. For example, drilling muds may be treated with barium sulfate (barite) or lead sulfide (galena) to increase their density.
The efficiency of the drilling process is related to the velocity of the mud flowing up the annular space between the borehole wall and the drill pipe. This velocity is in turn related to the viscosity, density and flow properties of the mud. In addition, the drilling mud viscosity is known to depend upon the quality, concentration and state of dispersion of the colloidal solids of the mud. As the drilling operation proceeds, the rheological properties of the mud may be adversely affected by such factors as the nature of the drilled strata, loss or gain of water to the mud, chemically-active contaminants that may flocculate the mud, mud pH, and, most importantly, the increasing temperatures and pressures encountered at deeper drilling depths. In order to maintain workable viscosities, the muds must be formulated to respond to varying circumstances and conditions encountered during use. Since improvements in efficiency are realized as the viscosity and density of a mud are increased, it is desirable to optimize drilling mud formulations to possess the highest viscosity and density workably feasible for a given formation at a given depth.
Elevated temperatures have a definite detrimental effect upon the drilling mud. This is particularly true for calcium-treated muds used to drill 12,000 foot or deeper wells, where the bottom hole temperature of the wells range from a low of 80.degree. F. to over 450.degree. F. Generally, the temperature increases with well depth, although the temperature-depth gradient varies widely from one geologic region to another. When the bottom hole temperature of a well reaches approximately 250.degree. F., the high pH, calcium-based muds become subject to high temperature flocculation and solidifcation, causing excessive viscosity increases that may lead to damage of the rotary drill bit. This viscous, gelled drilling mud also requires increased pumping pressures to effect circulation, if circulation is possible at all. The high temperature instability of the water-based drilling fluid or mud also may be aggravated by the presence of drilling mud contaminants such as gypsum, salt or cement.
Various naturally-occurring anionic polymers and their derivatives, such as mixed lignins, tannins, polyphosphates and lignosulfonates, have been used as thinners or dispersants in drilling operations to depths of up to about 12,000 feet. Unfortunately, these materials are unstable under the increased temperatures and pressures, possibly exceeding 450.degree. F. and 5000 psi, encountered upon deeper penetration of drilling operations. Upon exposure to such adverse conditions, the flow resistance of the drilling mud increases due to dispersant degradation and resultant excessive gel strengths and solidification. More energy and higher pressures are then required to pump the muds. Moreover, the instability of these additives usually requires their almost complete replacement during mud reconditioning before recirculation into the drill hole, leading to increased drilling operation costs and significant lost drill time. To overcome these disadvantages, more costly oil-based muds are sometimes chosen for drilling operations due to their ability to withstand repeated exposures to high temperatures and pressures.
A variety of drilling fluid additives have been proposed to stabilize and/or minimize the excessive viscosity increases of water-based drilling muds at high temperatures and pressures. U.S. Pat. No. 4,547,297 to Block discloses a high-temperature drilling mud stabilizer including a hydroxy-containing alumina component and a polyvinyl alcohol-aldehyde reaction product that is functional up to about 350.degree. F. Peiffer et al U.S. Pat. No. 4,537,688 discloses a terpolymer of t-butylstyrene-styrene-sodium styrene sulfonate as viscosification agents in oil-based drilling muds; and Gleason et al U.S. Pat. No. 4,518,510 discloses water-soluble sulfonated vinyl toluene-maleic anhydride copolymers as dispersants to provide muds with better high temperature and high pressure stability. U.S. Pat. No. 4,476,029 to Sy et al discloses the addition of a low molecular weight polyacrylic acid to a water-based bentonite clay drilling mud, also containing a water-loss controller and a weighting agent, for use at temperatures up to 500.degree. F. In U.S. Pat. Nos. 4,235,727 and 4,311,600, Firth discloses the use of a particular humate, related to but different from leonardite, as a drilling-mud thinner to impart high-temperature stability to clay-containing, water-based drilling muds. Other patents relating to drilling muds and drilling mud additives for increased high temperature and high pressure stability include U.S. Pat. Nos. 4,473,480; 4,411,800; 4,190,686; 3,091,588; 2,871,188; 2,860,104; and 2,836,556.
The present invention provides a bentonite-based drilling mud composition with improved high temperature and high pressure stability and rheological properties over the prior art water-based bentonite-containing drilling muds. Under high temperature and high pressure conditions, Sepiolite and Attapulgite are more commonly used in drilling muds than bentonite, since, at above approximately 250.degree. F., bentonite-based drilling muds will gel and rapidly increase in viscosity. However, it has been found that a drilling mud composition containing a high-yield bentonite, a low-yield or foundry grade bentonite and untreated leonardite unexpectedly can be used in a high-temperature drilling mud. By employing the drilling mud composition described herein, improved performance and efficiency in drilling operations are realized due to fewer stuck pipes, thereby resulting in fewer shutdowns, less energy required to pump the mud, and the ability for more efficient recirculation and reuse after repeated exposure to high temperature and high pressure conditions. Leonardite has been used as a drilling additive to stabilize and thin drilling liquids; however, the combination of leonardite with high-yield and low-yield bentonite to provide a drilling mud possessing such improved high temperature and high pressure properties is new and provides unexpected results in the art. These and other advantages of the present invention will be described more fully hereinafter.